(Proposal ID) S17A-014 (PI) Nakajima, Kimihiko (Proposal Title) The Nature of the Ionizing Spectrum in Early Star-Forming Galaxies (Abstract) The ionizing output of early star-forming galaxies is key to understanding whether they were responsible for cosmic reionization. Major uncertainties include the number of ionizing photons per UV luminosity (xi_{ion}) and the fraction that can escape, f_{esc}. Since neither can be directly observed for sources beyond z simeq 6, it is important to measure these parameters for suitable analogs at lower redshift. Following a successful pilot program, we propose deep MOSFIRE spectroscopy of a representative sample of z simeq 3.1 Ly-alpha emitting galaxies whose nebular [O{iii}] emission indicate they are useful analogs of early galaxies. HST time is now approved to complement this program by providing unique measures of Lyman continuum leakage and hence f_{esc}. Using the Hbeta line strength as a measure of xi_{ion} through recombination physics, we can correlate f_{esc}, xi_{ion} and the [O{iii}]/ [O{ii}] ratio, to verify suggestions from photoionization models that [O{iii}] intense galaxies beyond z=6 are capable of driving cosmic reionization. Our program will thus provide a valuable route to inferring the ionizing output of early galaxies ahead of the launch of James Webb Space Telescope (JWST).